1. Field of the Invention
This invention relates to an automatic focusing device for use in cameras.
2. Description of the Prior Art
Many devices of this kind have been proposed and are available commercially. In apparatuses using such devices, the range finding operation and the lens movement are performed either simultaneously or alternately. The former method introduces the likelihood of hunting when the environment is dim or when the radiation reflected from the object and impinging on a sensor of the range finder weak. Alternate range finding and lens movement causes a large difference in the speed of movement of the lens between the successive cycles of a drive operation. Thus, these methods have respective merits and demerits.
Some known devices respond to insufficient output from the range finder by causing the speed of movement of the lens to increase so that it takes but a short time to bring the lens into a range of positions for allowing the range finder to be responsive to the object. This shortens the time necessary to establish an in-focus condition. However, when the output of the range finder is sufficient, if there is a large discrepancy between the actual position of the lens and a position to which the lens must be moved to bring the object image into focus, it will take a long time to control the focusing adjustment on the basis of the accurate distance measurement. Therefore, it is desirable to drive the lens at a high speed even in this ease. However, because such prior known devices do not take the relationship between the lens start position and the object distance into account, the lens cannot be moved fast.
For the combination of a very far object and the lens start position set at a far object distance, consideration of the depth of field of the lens may make it unnecessary to move the lens to an accurate in-focus position, but merely requires focusing to the hyperfocal distance. However, even for such photographic situations, prior known devices cannot help but move the lens until the accurately adjusted position is reached. Therefore, the focusing operation takes a long time to complete.